1. Field of the Invention
The present invention relates to the production of a cold working tool steel for a long-life die having improved fatigue strength that is suitable as plastic cold working tools used under severe conditions, such as forming dies, forming rolls, and form rolling dies.
2. Description of the Prior Art
JIS-SKD11, a high carbon-high chromium steel, has hitherto been extensively used for cold working tools from the viewpoint of wear resistance. SKD11 (corresponding to AISI-D2) contains an M.sub.7 C.sub.3 type primary carbide composed mainly of chromium in a percentage area of 8 to 15%, thereby ensuring the wear resistance.
An advance of plastic working technology and an increase in strength of a material to be worked in recent years have increased a stress load applied to cold working tools used. This has increased situations with which SKD11 cannot cope due to unsatisfactory hardness and toughness. Specifically, for SKD11, which, upon tempering at a high temperature of 500.degree. C., has a hardness of 60 HRC, the wear resistance is still ensured, but the M.sub.7 C.sub.3 carbide is coarsened, unfavorably resulting in a lowered die life.
For this reason, inventions directed to various steels have been proposed from the viewpoint of improving the function of the material. These inventions are disclosed, for example, in Japanese Patent Laid-Open Nos. 201442/1989, 247357/1990, 277745/1990, 134136/1991, 156407/1993, and 212253/1994.
The invention disclosed in Japanese Patent Laid-Open No. 201442/1989 relates to a steel for a form rolling die, comprising by weight 0.90 to 1.35% of carbon, 0.70 to 1.4% of silicon, not more than 1.0% of manganese, not more than 0.004% of sulfur, 6.0 to 10.0% of chromium, 1.5 to 2.5%, in terms of molybdenum+tungsten/2, of at least one member selected from molybdenum and tungsten, and 0.15 to 2.5%, in terms of vanadium+niobium/2, of at least one member selected from vanadium and niobium with the balance consisting of iron, an M.sub.7 C.sub.3 carbide being present, in a quenched/tempered structure, in a percentage area of 2 to 9% with an MC carbide being present in a percentage area of not more than 2.5%. According to this invention, the percentage area and grain diameter of carbides are regulated with a view to improving mainly the toughness and preventing the propagation of cracks through a route of carbides distributed in a chain form.
The invention disclosed in Japanese Patent Laid-Open No. 247357/1990 relates to a steel for a form rolling die, comprising the constituents of the steel disclosed in Japanese Patent Laid-Open No. 201442/1989 and, in addition, not more than 0.13% in total of arsenic, tin, antimony, copper, lead, and bismuth. The invention disclosed in Japanese Patent Laid-Open No. 277745/1990 relates to a quenched/tempered structure wherein the percentage area in total of at least one member selected from MC type residual carbides and M.sub.6 C type residual carbides having a grain diameter of not less than 2 .mu.m is regulated to not more than 3% with the percentage area of M.sub.7 C.sub.3 type residual carbides having a grain diameter of not less than 2 .mu.m being regulated to not more than 1%. As with the invention disclosed in Japanese Patent Laid-Open No. 201442/1989, these inventions aim mainly to improve the toughness and to prevent the propagation of cracks through a route of carbides distributed in a chain form.
The invention disclosed in Japanese Patent Laid-Open No. 134136/1991 relates to a high-hardness, high-toughness cold working tool, comprising the constituents of the steel according to the invention disclosed in Japanese Patent Laid-Open No. 201442/1989 and, in addition, not more than 0.02% of phosphorus, not more than 0.005% of sulfur, not more than 30 ppm of oxygen, and not more than 300 ppm of nitrogen, wherein, in the quenched/tempered structure, the percentage area of M.sub.7 C.sub.3 type residual carbides having a grain diameter of not less than 2 .mu.m is not more than 8% and the percentage area in total of at least one member selected from MC type residual carbides and M.sub.6 C type residual carbides having a grain diameter of not less than 2 .mu.m is not more than 3%. The invention disclosed in Japanese Patent Laid-Open No. 156407/1993 relates to a steel for a high performance form rolling die wherein, upon quenching/tempering, a microstructure is developed with M.sub.7 C.sub.3 type primary carbides in a percentage area of not more than 4.0% and MC type primary carbides in a percentage area of not more than 0.5% being homogeneously dispersed in a matrix with the maximum grain diameter of the primary carbides being substantially not more than 20 .mu.m, and, when the steel is quenched from a temperature of from 1050-1100.degree. C. to 500.degree. C. at a cooling rate of 25.degree. C./min and then tempered at a high temperature, the hardness can be brought to not less than HRC 64. All of these inventions aim mainly to improve the toughness and to prevent the propagation of cracks through a route of carbides distributed in a chain form.
The invention disclosed in Japanese Patent Laid-Open No. 212253/1994 relates to a process for producing a cold working tool steel, characterized in that a steel product comprising by weight 0.75 to 1.75% of carbon, 0.5 to 3.0% of silicon, 0.1 to 2.0% of manganese, 5.0 to 11.0% of chromium, 1.3 to 5.0% of molybdenum, and 0.1 to 5.0% of vanadium, with the balance consisting of iron is tempered at a temperature of 450.degree. C. or above. This invention aims mainly to improve the toughness and to prevent the propagation of cracks through a route of carbides distributed in a chain form. Tempering at a high temperature of 450.degree. C. or above increases the secondary hardening hardness to markedly improve the service life and electrical discharge machinability of the cold working tool steel.
In all the above-described prior art techniques, the size of the carbide is regulated from the viewpoint of improving the toughness or the strength. That is, the above-described prior art techniques aim to prevent the accumulation of microdefects created by lack of primary carbides and to prevent the propagation of cracks through a route of large primary carbides distributed in a chain form.
An advance of plastic working technology and an increase in strength of a material to be worked in recent years have led to a strong demand for the development of a tool steel for a die having better wear resistance and fatigue resistance.